java的lambda表达式用法

过滤数组求第一个

Optional<WaterSensor> sensor = sensorService.getWaterSensors().stream().filter(s -> s.getId().equals(sensorId)).findFirst();

求list集合里面满足条件的个数

completeLogs.stream().filter(c -> c.getDate().equals(date) && (c.getGridChiefId() == g.getId())).count()

List转map

public static void main(String[] args) {
        List<Person> personList = new ArrayList<>();
        personList.add(new Person(1,"北京"));
        personList.add(new Person(2,"上海"));
        personList.add(new Person(3,"广州"));
        personList.add(new Person(4,"福建"));
        //转map
        Map<Integer, String> map = personList.stream().collect(
                Collectors.toMap(Person::getId, Person::getAddress)
        );
        System.out.println(map.toString());
    }

非常详细的lambda表达式案例

        List<Person> personList = new ArrayList<>();
        personList.add(new Person(1,"北京", "张三"));
        personList.add(new Person(2,"上海", "李四"));
        personList.add(new Person(3,"广州", "王五"));
        personList.add(new Person(4,"福建", "赵六"));
        personList.add(new Person(5,"福建", "范冰冰"));
        // 是否存在
        boolean matched =  personList.stream().anyMatch(person -> person.id > 2);
        System.out.println(matched);

        //找出id最大
        Optional<Person> maxIdOfPerson = personList.stream().max((e1, e2) -> Integer.compare(e1.getId(), e2.getId()));
        System.out.println("maxId:" + maxIdOfPerson.get().id);

        //找出姓名列表
        List<String> address = personList.stream().map(person -> person.getName()).collect(Collectors.toList());
        System.out.println("地址列表是" + address);

        //List转map
        Map<String, Person> personMap = personList.stream().collect(Collectors.toMap((key->key.getName()),(value-> value)));
        personMap.forEach((key, value) -> System.out.println(key));

        //统计地址是福建的个数
        long fjCount = personList.stream().filter(person -> person.getAddress().equals("福建")).count();
        System.out.println("福建的个数是：" + fjCount);

        //list转set
        Set<String> personSet = personList.stream().map(person -> person.getAddress()).distinct().collect(Collectors.toSet());
        System.out.println("personSet集合是：" + personSet);

        //查找地址为福建的人员
        Optional<Person> optionals = personList.stream().filter(person -> person.getAddress().equals("福建")).findAny();
        System.out.println(optionals.get().getName());

        //按照id降序列出所有人员
        List<Person> listDesc = personList.stream().sorted((e1, e2) -> Integer.compare(e2.getId(), e1.getId())).collect(Collectors.toList());
        listDesc.stream().forEach(person -> System.out.println("id降序人员" + person.getName()));

        //取id最高前两条人员信息
        List<Person> display2PersonList = personList.stream().sorted((e1, e2) -> Integer.compare(e2.getId(), e1.getId())).limit(2).collect(Collectors.toList());
        display2PersonList.stream().forEach(person -> System.out.println("id最高前两条人员名称" + person.getName()));

        //取平均id
        OptionalDouble averageId = personList.stream().mapToInt(c-> c.id).average();
        System.out.println("平均id是： "+ averageId.getAsDouble());

        //找到地址为福建的第一个人员
        Optional<Person> firstPerson = personList.stream().filter(person -> person.getAddress().equals("福建")).findFirst();
        System.out.println(firstPerson.get().getName());

求map集合里面值最大的对象

Map<String, Integer> map = new HashMap<>();
map.put("火狐1",1);
map.put("火狐4",10);
map.put("火狐3",3);
map.put("火狐2",12);
map.put("火狐22",12);
map.put("火狐222",12);
Optional<Map.Entry<String, Integer>> maxGrade = map.entrySet().stream()
                .filter(r -> r.getValue() != null)
                .max(Comparator.comparingInt(Map.Entry::getValue));